Yamamoto N, Satomi J, Tada Y, Harada M, Izumi Y, Nagahiro S, and Kaji R. Two-Layered Susceptibility Vessel Sign on 3-Tesla T2*-Weighted Imaging Is a Predictive Biomarker of Stroke Subtype. Stroke. 2014
As opposed to myocardial infarction, stroke is a heterogenous entity. The burning question that a vascular neurologist seeks to answer when a patient presents with acute ischemic stroke is where is the acute intracranial or extra cranial occlusion. Where did the clot come from? Was it an in situ thrombus atop an underlying atherosclerotic plaque that ruptured, or did it come from a central embolic source, vis-a-vis the heart. Depending on where a thrombus is formed, it may be white, red or mixed. Traditionally, it has been thought that white thrombi develop on ruptured plaques and comprise of aggregated platelets, whereas red thrombi form in cardiac and venous systems, and are rich in fibrin and trapped erythrocytes. However, recent histopathological studies looking at clots retrieved by endovascular retriever devices failed to show an obvious association between clot composition and stroke mechanism.
Over the years, there have been several studies trying to identify radiologic biomarkers to identify clot composition accurately, thereby ultimately attempting to establish the mechanism. Animal studies also have shown that erythrocyte rich thrombi may be more amenable to thrombolysis by tPA as opposed to fibrin rich ones. Hyperdense vessel sign on CT is one such marker, and has been associated with erythrocyte rich clots.
The sequence of degradation from oxyhemoglobin to paramagnetic deoxyhemoglobin, methemoglobin, and hemosiderin within the hemoglobin components in trapped erythrocytes in a clot can be exploited by MRI T2*-weighted gradient echo imaging (GRE), whereby red thrombi in occlusive vessels may be seen as hypointense signals within vascular wall . This has been called susceptibility vessel sign. SVS has been associated with cardioembolic stroke mechanism and subsequent recanalization.
In the current study, Yamamoto et al. investigate if two-layered SVS on 3T T2*-weighted imaging (hypo intense core surrounded by hyper intense rim) also associated with cardioembolic etiology. 132 consecutive patients with ICA or MCA occlusion were included, with an mean age of 74 years. As opposed to prior studies, traditional SVS was not associated with cardioembolic mechanism, but two-layered-SVS was found to have high specificity and positive predictive value for cardioembolism. One of the major drawbacks of the study is that no histological analysis of the clots was done. Also, very few stroke centers have access to 3T MRIs in the acute setting. Thus, these findings have poor external validity to most clinical sites. Regardless, it builds up on the prior studies and makes a case for us to pay more attention to the vessels on GRE in patients with large vessel occlusions, especially when the source is elusive. MRI technology has brought stroke care a long way since its inception. And it may be our best noninvasive tool at showing us clot composition moving forward.